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Has the design-flaw with the Boeing 737's cabin-pressure switch been fixed yet?
At 42.21 minutes of this video about the crash of Helios' Flight 522 (a Boeing 737) caused by cabin depressurization, it was established that a major cause of the crash was a design flaw -- the main wire and the backup of a wire running from the cockpit to the depressurization valve runs through the same channel. So, if one wire burns up and fails, they both do. This is evidently contrary to safe design practices. Also, it seems that the cockpit instruments in the cockpit do not give sufficient indication during the preflight and takeoff checks that this problem exists.
From this video, it appears that this problem has not been addressed by Boeing. Is this still the position on current date?

Using the term "depressurization" with regard to the Helios crash is a little dubious... the cabin never pressurized in the first place.  While various factors contributed to the crew's mistake, the bottom line is that they had a fully functional 737, albeit with one switch in an uncommon position (that they failed to catch & correct). Compared with every other 737 out there, there was nothing wrong with theirs that caused the crash. Discussions of wiring & backup wiring & etc is all hypothetical, and might explain another accident. This one, however, is entirely explained by human factors.

The Wikipedia article on Helios Airways Flight 522 also mentions this wiring issue, and cites a separate report regarding a May 2003 incident in which a 737 had issues with pressurization. During the investigation it was discovered that wiring had shorted and burned in a particular bundle of wires, called a loom, which was important for control of pressurization. The AAIB issued a safety recommendation regarding this issue:

Safety Recommendation 2004-33
It is recommended that in order to prevent failure of the cabin pressure control system in the event of damage to wiring loom W298, the Boeing Commercial Airplanes should consider, on the Boeing 737-436 and similarly configured models, separating or protecting the wiring associated with the different modes of operation of this system, which connects the cabin pressure controller to the rear outflow valve, such that any single point failure of the loom would not result in effective failure of the pressurisation control system.

In their 2006 progress report, the AAIB followed up on this recommendation:

After a review of the issue and the service history of the pressurization system for the 737-100 thru - 500 model airplane, Boeing felt that modifications of the wiring with the intent of separating the outflow valve functions consistent with the subject safety recommendation would not significantly contribute towards a reduction in the overall rate of depressurization events. Their rationale was based on the basic design philosophy and concept of the original system which includes:
1) A single, mechanical outflow valve which is controlled by a single multi-mode analog pressure controller and driven by independent AC or DC powered electrical motors. Each motor can also be manually commanded open and closed via separate switch on the pressure control panel.
2) In the event of a complete power/control loss (e.g. - electrical interruption or damaged wiring) either motor will remain in last commanded position. The airplane is equipped with an independent aural warning system (cabin above 10,000 feet) and visual cues (cabin above 14,000 feet) should this condition lead to a cabin pressure loss.
3) The airplane is equipped with both fixed and portable oxygen supply systems for passengers and
  crew.
4) If pressure cannot be reliably controlled/restored within a short period of time, the flight crew will utilize procedures (such as rapid reduction in altitude, declaration of emergency and landing at nearest suitable airport) which mitigate the exposure time to low pressure.
Status - Rejected - closed

So the rationale is that if the system fails, the crew is alerted and may take appropriate action. Note the following regarding an AD published in 2011 with a deadline of March 2014:

The FAA released Airworthiness Directive AD-2011-03-14 requiring all Boeing 737-100, -200, -300, -400 and -500 types to install two additional warning lights, one for takeoff configuration and one for cabin altitude warning, and introduce updated documentation within 36 months following March 14th 2011.

Bottom line: The design is good, it allows to control each valve with two different motors and circuits, and in addition it can be manually operated. In case both circuits are damaged, an already low likelihood event, the crew is still alerted and can operate the valve manually and/or proceed to emergency descent. It's enough to make the alert redundant to respond to the accident.

I would like to add that the linked video deviates in major ways from the truth - or at least from the Greek accident board's assessment.
Firstly, the final report of the Air Accident Investigation  & Aviation Safety Board (AAIASB) does not mention the wiring problems at all. Nor does it mention anyone noticing a "burning smell" as is said in the video. If they ever considered it a possibility it had obviously been ruled out before the final report was issued. In fact the report does reference several prior 737 pressurization incidents, each of which were attributed to failures in the operation of the controls and not any malfunction. Their recommendations all pertain to changes in the wording of procedures and changes in the cockpit warnings, i.e. a different warning horn and more noticeable indicators on the panel.
The firm conclusion of the investigation was that the pressurization switch had been left in the manual position and the pilots misinterpreted the warning horn. This was evidenced by
1) the position of the switch as it was found in the wreckage

The cabin  pressure mode  selector was confirmed  to  be  in  the MAN (manual)  position  by  physical witness marks  on the  mode  selector input  shaft  and by electrical continuity  checks in  the mode select  circuitry. *

2) the Non-Volatile Memory (NVM) chip inside the controller

The NVM recording  showed  that on  the  accident flight, the cabin  pressure  control  system was being operated  in  the  manual mode. *

3) conversations with ground personnel

The Helios  Dispatcher  reported  that the  Captain  referred  to  the  Takeoff  Configuration horn,  and  then the Equipment  Cooling lights. *

The video also said that the the F-16 pilots saw two people struggling in the cockpit when they arrived. I have no idea where they got this (made it up maybe?) because the investigation states,

When  the flight HCY522 was intercepted  by the F-16s,  the F-16 lead  pilot reported that there  was no  visible  damage to the Boeing  737 aircraft, that the Captain’s  seat was vacant,  the person  in  the  First  Officer’s seat  was not wearing  an  oxygen mask  and  was  slumped  over  the controls,  and some  seated passengers in the cabin were observed wearing oxygen  masks. *

It appears to me that the linked video, which is titled "Aircrash Unsolved: The mystery of Helios 522" is taking a lot of liberties with reality to make a dramatic "mystery" where no real mystery exists. The fact that the YouTube account that posted it misrepresents it as an episode of "Air Crash Investigation" is bothersome. The latter show usually does a much better job of being accurate.
* Taken from the AAIASB final report.

I realize this is really not an answer to the actual question. Fooot's answer covers the question very well. This should really be placed as a comment but it is way too long for comments.


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Do jackscrews suffer from blowdown?
With hydraulic control surfaces, at high airspeeds, there is a point where aerodynamic loads exceed the capability of the actuators. This limits control authority and can result in the control surface not being in the position commanded by the pilot.
Jackscrews are commonly used for horizontal stabilizer control. I know they can jam and be difficult to move under high loads, but can they be blown back from their set position or do they stay put?

Acme screw type screw jacks with the square threads, as used in stab trim systems (as opposed to a recirculating ball screw), are usually inherently irreversible because of the higher friction of the direct sliding contact of the square sided threads vs a rolling ball interface (as a sliding interface, it's totally dependent on the grease to keep friction in check).
This is why the trim screw jack of the 737 becomes extremely difficult to move manually when the jack is heavily loaded, even though the movement input is coming from input side that is supposed to be the low effort side, requiring a technique of unloading it with elevator inputs to manually trim when the airplane's actual speed and trim speed are far apart.
Ballscrew type jacks however, typically used for flap actuators, have very low internal friction and very high efficiency thanks to the ball interface between the "nut" and screw (rolling, not sliding).  As a result they can be back-driven more easily; how easily depends on the gear reduction within the worm drive gearbox.
Ballscrew operated flap systems typically require friction brakes in the drive line, or anti-backdrive devices incorporated internally in each actuator (basically a clutch device that is disengaged only when the torque is coming from the input side), to prevent a flap surface that is disconnected from its drive motor from creeping up from air loads.
Acme screw actuators also have internal brakes but these mainly function to lock an inoperative motor when there are dual motors, so that the live motor can't backdrive the dead one instead of the downstream gear train.  They also serve to lock the acme screw when both motors are off, but this isn't as critical with an acme screw.

I thought it was pretty clear.  Yes square threads have less friction than tapered ones.  Read it carefully. I'm talking about a sliding interface vs a roller interface.  A ball screw sees no sliding of surfaces, it's a spiral ball bearing in effect.  Ballscrews only need grease to keep water out and to lubricate the balls where they touch each other.  Remove the grease and it's efficiency is only marginally reduced.  An acme thread is two spiral surfaces sliding past each other and needs a grease film to function at all.

I've made some changes that may help.  Interesting find there. Stab jacks on transport a/c use dual interleaved threads (it looks like one screw but it's actually two intermeshed) to allow a nut with two separate thread load paths.  Not sure that can be done with a recirculating ball system, so that may be why you don't see that type in service.

would a sufficiently steep worm gear not prevent actuation from the load side all of its own?

Yes if the back drive loads are low enough, the gearset internal friction is high enough and there is a very large gear reduction.  This often isn't practical in a ballscrew design where you are going for high efficiency and in something like a wing flap there is also a lot of vibration to help airloads cheat things along.  Very high efficiency actuators like ballscrews and planetary rotary actuators as typically used for slat systems almost always need internal brakes or drive line brakes.

Boeing does not use acme screws on H. Stabs. They are all ball screws with brakes, all the way back the 707 and KC-135. Douglas used acme screws, however.

Screws can be either self-locking or overhauling. Jackscrews used for stabilizer control are designed to be self-locking, since their purpose is to make the stabilizer adjustable, but prevent it from moving by itself.
A jackscrew (or screw jack) is pretty much by definition a self-locking screw.
It should be noted that vibrations can induce travel in screws, even those that are self -locking under static load by desing. I was, however, unable to find a study regarding such scenarios in jackscrews applied in aircrafts.
It is reasonable to assume that jackscrews used to adjust the stabilizers are designed in such a way, that the trim system as a whole is not prone to moving on its own in any foreseeable scenario.
Wikipedia: Screws - self-locking property

No, a jackscrew is not self-locking.  Screws are only "self locking" to a first approximation in static applications.  The addition of continual or severe vibration will facilitate the loosening or "back driving" of pretty much any screw.  This is why screws and bolts on aircraft have to be secured with "locking" compounds, or safety wire, or similar mechanism.  Jackscrews generally have some kind of brake scheme that is applied when the screw is not commanded to motion.

That is a good point @Brock Adams, I have to dig in deeper and most propably amend my answer. Do you have any sources at hand?

Not at hand, just a few years working on various aviation devices.

Limited control authority can be a design feature. Full surface deflection at high airspeed may impose high load factors which exceed the structural design strength of the airframe, in which case the control surface authority can be limited by the maximum actuator force.
Jackscrew actuators can be designed such that they are back driven. Reversibility of the load is a function of:

Backdrive load.
Thread friction.
Screw thread pitch.

There is a self-locking pitch limit: between zero and this limit, the screw is irreversible (self locking). The best known example of this is found in nuts and bolts, which have the cross sectional triangular thread shape resulting in the highest friction. Note that:

The bolt becomes more difficult to turn with increasing tension: the friction is still a function of the back load.
All bolts in aircraft must have a locking feature to prevent unlocking due to vibrational friction fluctuations, as mentioned in a comment from @BrockAdams.

Drive screws are mostly not designed to be irreversible, most threads for drive shafts have a rectangular thread and high pitch in order to minimise friction and maximise efficiency.
The stabiliser pitch screw in an aircraft is subject to varying loads, both in a positive and a negative direction, and can therefore not be designed such that it is self locking due to load and friction. If that feature must be designed in, a separate lock must be manufactured, with redundancy features to prevent uncommanded lock in undesired situations.

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Is there a way to find out which runway is used by which flight on a particular day/date?
For example, i want to know which runway was used by NH 829 that flies from NRT to BOM (Narita-Tokyo to Bombay-Mumbai)
For flight tracking, I know and use these websites/apps - FlightAware, FlightRadar, FlightStats, Aviability, FlightMapper, Planefinder and Kvikr.

https://www.flightradar24.com/data/flights/nh829
https://flightaware.com/live/flight/ANA829
https://www.flightstats.com/v2/flight-tracker/NH/829
https://planefinder.net/data/flight/NH829
https://www.flightera.net/flight/All+Nippon+Airways/NH829
https://info.flightmapper.net/flight/ANA_NH_829
https://www.radarbox24.com/data/flights/NH829
https://www.airportia.com/flights/nh829/tokyo/mumbai/
https://www.planemapper.com/flights/NH829
https://kvikr.com/flight/nh829_ana

But none of them provide information about runway used for takeoff or landing at the source or destination airports respectively. I am looking for a publicly accessible, preferably free to use website/smartphone app/API or even a historical dataset (recent flights not those from the previous decade)
Is such information kept a secret (intentionally) or is it just that no one displays it, probably stored in the database. I understand that realtime information may not be available, but for past flights, such data should be available. I guess there could be country-specific restrictions too. But i want to see as much public data for any flight as available.
My ultimate goal is to find out which runway is used by LH717/LH716 and QF25/QF26 for takeoff and landing at HND (Haneda-Tokyo), so that i can get a better view of the Boeing 747 from as close as possible.

If getting close to the aircraft is your goal a historical dataset isn't really going to help much as the runway choice comes down to which way the wind is blowing.

i agree. historical dataset may be useless. I need MRU(most recently used) data, the reason i used "historical dataset" is - i thought i could find some patterns in it, but flight patterns are as uncertain as winds, yeah. thanks. I guess i can calculate seasonal probability at least from the historical dataset though. (say i could get list of flights and how often QF25/26 or LH717/716 are using runway D "05/23" at HND)

@computingfreak I am not familiar with operations at HND, but at many airports if trying to spot a particular aircraft, you’d be there well before the aircraft of interest arrives. There is a very good chance it will use the same runway as everyone else of a comparable type or weight class or “internationality”, depending on how the airport works, I would think?

It confuses me that you say the flight tracker do not show which runways were used. I usually look at fightradar24, and its position tracks are almost always precise enough to make it unambiguous which runway a tracked flight departed from (or arrived on). At airports with good coverage even the taxi route from the stand to the departure runway is plainly visible.

(Guessing the) Runway Selection
For spotting airplanes, you want to know which are the current active runways. The runway history of planes or flights shouldn't matter much, unless your plane of interest is limited in what runways it can use.
Large airports have many parallel runways. In general, outsiders will not have enough information at hand to predict the active runways. The factors that are considered for the selection of active runways are the following:

Aircraft take-off and land into the wind, or with minimal tail wind.
  Based on the wind direction, air traffic control will decide which
  runway is to be used at any given time.
Larger airports tend to have more than one runway, so that a runway is
  always available depending on the wind direction. Airports with just
  one runway are generally constructed so that the runway is aligned
  with the prevailing wind.
Runway selection is monitored at all times, as weather conditions can
  quickly change. Every runway has a wind indicator known as an
  ‘anemometer’, and wind observations contribute to the runway selection
  decision. Pilot reports of upper level winds can also impact on runway
  selection.
When a runway is selected, it needs to be available for an extended
  period of time to allow pilots to plan their descent, approach and
  landing. As this involves anticipation of developing trends, aircraft
  may continue to land on a runway for a period when weather conditions
  at a local level no longer appear to warrant it.
Other factors that air traffic control will take into consideration
  when deciding which runway to use include:

the number and type of aircraft programmed for the airport
length of runway(s)
weather conditions (both present and forecast); including wind velocity and gradient, wind shear, wake turbulence effects and
  position of the sun
availability of approach aids in poor visibility conditions
location of other aircraft
taxiing distances, including availability of taxiways
braking conditions.

Some airports also have ‘preferred runway’ systems. This means that if
  wind conditions, workload and traffic conditions permit, a particular
  runway will be used to move traffic as efficiently as possible to
  reduce the noise impact over residential areas.

This text is quoted from a short informational document on runway selection published by Airservices Australia.
ATC radio communication
It is possible to listen to ATC radio communications with a handheld scanner (always check if this is legal in your country!), or via the internet (LiveATC). There are different channels including Departure, Approach, Ground, and ATIS.
ATIS
Busy airports are usually equipped with Automatic Terminal Information Service. ATIS continuously broadcasts relevant information about the airport and immediate surroundings for pilots. This includes the actual runway in use. One can receive ATIS via VHF, or via phone. Frequencies and phone numbers are published in different ways. Airnav.com might be useful for the USA. LiveATC provides streaming of ATIS.
Ground
Aircraft are cleared for taxi to a runway on the Ground channel.
Approach
Aircraft receive their instructions and clearance for a runway on the Approach channel.
Websites
Airports
Some airports are publishing their actual active runways on a website. For instance:

Amsterdam (Schiphol): "Baangebruik Schiphol".

This airport always assigns 1 active runway for landing and 1 active runway for starting. In the rush times, it opens an additional runway for landing or starting. In the snapshot above, the 2 runways for landing are red, the single runway for starting is blue.

Brussels: Runways in use

There are probably not that many airports publishing this information to non-pilots, but surely there are more. Please comment if you know any other!
Flight Tracking
If there is no information available from the airport directly, you can always use flight tracking web sites like the ones you have listed in your question to observe which are the current active runways, for starting and for landing respectively. Keeping in mind that the assignment of active runways is not random, it is always reasonable to guess that the next aircraft will use the same runway as the previous ones. For approaching aircraft it is often possible to see if it is directly pointing to a runway, or if it follows a pattern to a runway that other aircraft have taken before.
Standard patterns and Operational procedures
If interested and willing to dig a bit deeper, you can always find and study standard approach patterns and operational procedures of your airport, e.g.

Operational Procedures at Tokyo International Airport (Haneda)
search the internet for "RJTT SID STAR"

In combination with flight tracking web sites, this will allow you to recognize the usual patterns, and predict earlier where a tracked plane is going to land.

It may be worth noting some parameters that determine what runways are active to begin with, for example wind direction.

You can also look at FlightRadar24 and figure out the active runways based on current air traffic: just watch where aircraft are landing and taking off.

Thanks @bogl, the documents at http://www.uvairlines.com/admin/resources/RJTT.pdf, https://yinlei.org/x-plane10/doc/AIP-J/RJTT_Tokyo_INTL.pdf, https://www.twin.ne.jp/~watanabe/satjpn/files/RJTT.pdf and http://minietonrailroad.org/charts/RJTT_Tokyo.pdf based on "RJTT SID STAR" and the Operational Procedures at Tokyo International Airport (Haneda) document seem to answer my questions, will read them and find out rules for 747 aircraft, possibly eliminating unusable runways due to aircraft type/size.

you forgot THE major factor in runway selection on ever more airports (and specifically EHAM): noise abatement. Schiphol is well known for selecting the runways to use almost exclusively based on noise abatement requirements unless winds are such that use of other runways is mandated to prevent aircraft going over their crosswind limits. This has caused at least one aircraft to have an unscheduled runway vacation (a Transavia 757) when attempting to land in heavy crosswinds because the optimal runway was closed by orders of the ministry of the environment.

FlightAware now provides Active and Actual runway information on the flight page. Disclaimer: I work at FlightAware but was not involved in this feature, so can't provide any further details.

where/how? sample link/screenshot please

On the right side of the track map there's a box that includes flight information including runways and whether they're actual or active, if that information is available. https://imgur.com/YKRNurm

The choice of runway depends on wind and other factors and will change frequently. The dominant runway(s) can change several times a day if the wind is changing a lot.
You could potentially detect which runway set was being used by evaluating FlightAware information, but that would require programming and geographic computations. There is no service that attempts to do this kind of computation currently.
Even if you knew the dominant landing direction, if there are parallel runways, there would be no way to determine which of the two was being used for any particular flight because that changes more or less randomly on a flight by flight basis depending on the local traffic needs.

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How to estimate runway length for airport master planning?
I use the manufacturer aiport planning manuals. But they are incomplete (unless for Boeing). Airbus and Embraer for example, only present range X weight charts for MTOW.
Is there a free (or affordable) software or good charts for this estimation?
I'm interested in airport planning.

Not sure what you are after, Airbus and Embraer do include take-off field length charts in their airport manuals.

The airport planning manuals from Airbus do include take-off field length charts for various weights, temperature and airport elevation.
Those of Embraer contain similar charts.


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Why jet engines can differ in shape?
I have seen the shape of the A320s engine and for example the B737 engine, its different, I want to know why its different? is it just for designing?

The reason for it is because the 737 has a very unique shape and that shape is because it needs more ground clearance.

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Why is tethered hover testing performed on helicopters?
The KGUC website notes that they have a tether for helicopter testing. From what I can tell this testing has something to do with hot and high conditions, but I don't know what the purpose is of testing with a tether. Here is a video of testing being performed. What is tether testing for?

Interesting question, I can see the load cell mounted on the tether, they may be measuring lift at various configurations. It would be interesting to know how they compensate the readings for being so close to the ground. Enstrom Helicopter has a manufacturing facility in my hometown but I haven't noticed any kind of test pad like this at their facility.

@mins I didn't even think of reading the video description. [smacks forehead]

The cable is attached to the helicopter with a tensionometer.  This can be used to vary the effective weight of the helicopter by varying the tension applied to the cable.
The cable length can also be varied from 0 height, to hovering in ground effect and  hovering out of ground effect.
This test enables data to be gathered quickly for the helicopter performance from minimum to maximum weight on the ground, IGE hover and OGE hover without the pilot having to control height (since the cable does this) and with quick and easy changes to the effective weight without having to add ballast to the helicopter for each weight.
This test is a simple and quick, albeit risky, way to gather data on hover and climb performance throughout the weight range.

Is this something done for a specific airframe?

It's a general technique AFAIK.  There might be some limitations such as is there a hard point on the underside of the aircraft (like a cargo hook) that can take the load?

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Why was the Yak-9U covered with Bakelite?
According to Wikipedia the entire Yak-9U was covered with Bakelite (not sure if it was just that variant, the page isn't clear). What was the reason for this and was Bakelite used as a coating on any other aircraft?

The aircraft used bakelite impregnated wooded stressed skin (basically molded Bakelite birch plywood). For example, airvectors notes:

In the late spring of 1942, the increased availability of aviation aluminum alloys led to the development of a reconnaissance variant of the Yak-7 with a new wing, featuring metal H-section spars with Bakelite-impregnated wood skinning.

This development variant became the Yak- 9. Yakolev had used this method construction earlier in Yak- 2. This method of construction was used in a number of other aircraft too. For example, the Mikoyan-Gurevich MiG-3 used a combination of Aluminum and bakelite infused plywood:

... the fighter was of mixed construction: the forward and central sections were of welded steel tubes construction covered with Dural panels, the rear- and tail- sections were semi-monocoque structures with a skin of bakelite-impregnated plywood over wooden longerons and stringers.
The wing was a stressed-skin structure: the flat center section, which was tapered in thickness and chord, had a flush-riveted Dural skin over a steel and Dural structure, while the dihedraled outer wing panels, which were also tapered in thickness and chord, possessed a skinning of bakelite-impregnated plywood over a wood structure.

These are early composites which increased strength and water resistance. This method was especially attractive to the Soviets due to the early loss of industry (Aluminum plants) and wartime scarcities.
This method of construction was used in a number of other aircraft with various modifications, like the Fokker DXXI among others. This method was used, albeit on a smaller scale in the US too (There was little incentive there to use wood for aircraft construction).

I would think Bakelite would be way too brittle to use on wings, even if it's impregnated into wood. One good hard turn and it'd be full of cracks


text:
Why bother having different engine options on one aircraft?
When an airline purchases a new passenger jet, they often have a choice of engines (from different manufacturers). Why is this?
I would expect that one of the engine options is "best" and that all customers would buy that option. Thus it wouldn't be worthwhile to certify and offer the other (unwanted) options. Evidently this is not the case, so the question is why.
I can think of two possible reasons for this:

Different customers may have different requirements (e.g. shorthaul versus longhaul routes, operating in hot or cold environments) and different engine options have differing performance (e.g. better cruise efficiency versus climb efficiency or better performance in hot/cold temperatures). But I can't imagine that the requirements would be significantly different and, if they were very different, they would buy a totally different plane, not just a different engine.
Business/legal considerations: Perhaps airlines have contracts with engine manufacturers separate from aircraft manufacturers. e.g. British Airways may have a special relationship with Rolls Royce (possibly due to trade tariffs). But, again, I would imagine that airlines would always seek the most efficient engines, rather than signing such contracts.

Are either (or both) of these reasons right or is there a totally different reason?

Why would you expect one option to be "best" in all respects?

@DavidRicherby: That is essentially my question: Why do different customers think different engines are best for them? I would expect that the customers all have similar requirements.

Engines require a significant amount of specialist maintenence through their life. This means that an airline must train technicians to work on each kind of engine it runs, and maintain stocks of spares for these engines. Engines must also be periodically returned to the manufacturer for rebuilding and refurbishment.
Engines from the same manufacturer will have a certain commonality in terms of parts and maintenance techniques, which means that inventory and training costs will be reduced. Also, because engine maintenance requires significant interaction with the manufacturer (for refurbishing but also for parts and training), airlines wish to be able to chose which company that will be and also to have the opportunity to build up a business relationship with them. Airlines are likely to feel that they'll get a better deal from a manufacturer that supplies them with a large number of engines.
Even if an airline chooses to buy a small number of engines from a supplier, there's still hope of getting a good deal because they can threaten to take their business elsewhere. But if only one engine is available for a particular plane, the choice is "Buy from supplier X or good luck with that gigantic glider", and the customer has very little negotiating power.

Exactly. Managing a maintenance contract costs money.  Managing two contracts costs more money than one big contract. They are a long term business partnership.  It's a big effort on both sides with a lot of on-site support.  And it's important to note that engine maintenance contracts are where the engine manufacturers make their profit.

I would imagine that there are factors like airline maintenance crew familiarity, national identity (think British Airways switching away from Rolls-Royce engines entirely to only GENX), and perceived vs. real engine reliability and long-term operational costs.  There are carriers (and people, the current US president's personal plane being an example of this) who won't fly any engine except the RR.  I would imagine that when the next VC-XX plane comes along to replace the 747-200's (which I believe have CFMs in them), the choice will be for either CFM or GE over Rolls.  To chose engines which are designed, manufactured, and (primarily) maintained by a non-US company would not exactly look good on the jet which carries the country's chief executive.
The other issues are around very real things like the Trent 1000's problems on the 787.  RR has now stated they don't expect to make any money on the deals as the engines are wearing out much faster than expected.  GE, on the other hand, seems to be OK at this point.  The 787 has had its issues, but they aren't completely germane to the question asked and are more around the fact that the 787 is a completely new way to build an airplane for Boeing.
Now, if you want a book answer (there are many), here's a 6 page long, but meaty paper which covers it: http://ieomsociety.org/ieom2011/pdfs/IEOM109.pdf.  In short, short-haul or "budget airlines" tend to have the same engine across their fleet to save on maintenance whereas the "flag" carriers are much more diverse.  All this goes to my first point of "airline maintenance crew familiarity".  The flag carriers can afford to have many people trained on many types, the budget carriers, not so much.  I'll leave you with the prime example of this -- Southwest Airlines.  They are so budget conscious that they fly the exact same engine model and type on ALL of their 737s and all 737s are identical with-in the extent made possible by Boeing.  This means things like the new HUD display that most 737s have is not allowed to be used in any SWA jet, even when it's equipped.  This makes the crew able to go from plane 1 to 2 to 3 to 974 and they will all look the same and will all be type-rated the same.  The maintenance departments train on only one finite set of parts for one type of engine with the same specs.

Welcome to Aviation.SE! A nice first answer.

Airlines don't want to buy "engines" at all. From the airlines' point of view, they are just horribly complicated things that they don't understand, and they don't want to spend their own money learning anything more than the information in the Pilot's Handbook!
What airlines really want is "something to power their planes". These days, the engines are often leased, not bought, as part of a "power generation package" which includes long term performance guarantees, scheduled maintenance, etc.
Packages like that are often tailored to very specific requirements. For example in the middle east, some airlines prefer to use higher power settings for the climb after takeoff, so they can get above the airspace that is controlled by a neighbouring country's ATC and avoid delays, extra fuel burn while restricted to low altitude operation, etc.
The only physical difference between two "different versions" of a particular engine type, with different advertised power ratings, may the electronic engine control system. In that situation, the airline might negotiate a "total power generation" deal where they only pay for the "higher rated" engine when they actually use it.
In that sort of business environment, the idea of a "best" engine for every customer is much too simplistic.

Interesting. I had not heard about engine leasing or the pricing model, but I guess that makes sense.

ymb1 answer is good. I would add that often there are political reasons: for example currently you cannot sell GE engine to any Russian company for the trade sanction beetween USA and Russia. If you sell an helicopter or airplane with your GE engine... you simply don't. The solution is to use another engine that is not made in USA.

Good point. However, using a Rolls Royce engine wouldn't bypass USA sanctions if the plane was made by Boeing.

@Thomas -- the US sanctions against Russia are focused on direct military investment AIUI (the Russian airline fleet is mostly Western aircraft nowadays even)

During the cold war when Russia were developing the Tu-144, they made a request to Lucas Technologies for access to their engine management system, and BAC for help in improving the air intakes of the aircraft. The British Government vetoed any help, as they felt that the technology could be used in military applications. Whether that would've happened or not is unclear, but it's interesting to note that even civil applications can be stopped on the grounds of potential military involvement.

I would expect that one of the engine options is "best" and that all customers would buy that option.

"Best" is a subjective concept.
What is best for one airline might not be evaluated the same way by another airline. One airline might define "best" as whichever viable option is cheaper at the time they make the purchase. Another airline might define "best" based on fuel efficiency, total power output, total life, or some other metric or set of metrics that feed into a total cost of ownership formula. Another airline might define "best" as matching the long-term supplier they happen to have standardized with. Or the engine vendor that does the best job schmoozing the airline executives (really).
Having multiple engine options allows the airplane to work with whichever option the airline customer deems best, and thus increases the value and market for the plane vs only one possible engine. It also increases the utility of the place in case of a supply problem with one engine type.
The idea that "best is subjective" applies to many areas of life beyond just airline economics. It speaks to value choices that all of us make, often subconsciously.

Engines for an aircraft are more consumables than proper constituent. They will be maintained, overhauled and replaced many times during the lifetime of the airframe, all of which require great number of deals and purchases from the engine maker. The airline may want to either strengthen the bond with one engine manufacturer to benefit from synergies, or the exact opposite, diversify engine sources in order to strenghten negotiating position.
So choice of "GE vs RR" for a plane is more akin to "diesel vs gasoline" for a car.
It's not #1, it's #2. But less to do with tariffs and more with pre-existing business deals.


text:
Is if safe/legal for an aircraft to return to cruising altitude after passenger oxygen masks have been deployed?
I was reading reports [1,2,3,4] about a recent Air China flight where one of the pilots was allegedly smoking and, in an effort to prevent the smoke circulating to the cabin, accidentally de-pressurised the aircraft. The passenger oxygen masks were deployed and the crew performed an emergency descent. Eventually the problem was rectified and, after the cabin re-pressurised, the plane climbed again and completed the flight.
The bolded parts of this story are what I'm asking about. If the oxygen masks have been deployed and the plane returns to cruising altitude, then there is no supplementary oxygen available for passengers in the event of a second de-pressurisation incident. This seems unsafe. (I recall reading that the oxygen supply only lasts 12 minutes and is intended only to last for the duration of an emergency descent.)
What are the rules around this situation? Can an aircraft safely/legally climb again after passenger oxygen masks have been deployed?

There is no definite answer that I can find as the entire topic is debatable

Why the downvote? @FallenUser given how procedure-oriented commercial aviation is, I would be surprised if there is no answer to this question.

Well, guess what! There are 2 answers right now!

Once the masks have deployed and any of them have been activated, you no longer have an emergency oxygen supply for all the passengers. Not having that, going above FL250 (may vary in jurisdictions besides the FAA) is not allowed. Up to 250, emergency O2 isn't required for the passengers, since you could descend quickly enough to a safe altitude in the event of a depressurization. Above that, though, having the masks available (and no, hanging down doesn't quite count) is required, both for legality and for safety.
To climb back up to a cruise altitude in the 30's is a pretty significant misjudgement, given the facts as they've been presented to date (which may have significant mistakes in the reporting). Up there with depressurizing the aircraft when you meant to turn off the Recirc fan. Or vaping in the cockpit. Or allowing vaping in the cockpit. Or not turning the packs back on immediately when your FO has incorrectly turned them off.
Sometimes you just have a bad day. And then there are some people who need to switch career paths, because they aren't cut out for the one they're in. We all have our own opinions about what the case is here...

No you wouldn't be able to legally climb above 10000ft once the masks have deployed, not even in China. There are a bit behind on Crew Resource Management theory and occasionally, wild and crazy things can happen there, although the CAAC is doing its best to crack down as the article shows.
One problem however is that in the zeal of the authorities to achieve western standards, they have an extremely punitive enforcement regime, both on the pilot and maintenance side, which creates very powerful incentives to hide mistakes. Similar problem in Russia.

can you please provide sources for the first statement?

Hmmm.. what if there was insufficient fuel for nearest-runway diversion at 10000?  Could that be an issue on ocean flights?

@Federico Check out the website https://onemileatatime.boardingarea.com/2018/07/13/vaping-air-china-pilot/. You might have missed what it said

@MartinJames Fuel planning regulations are in place to cover this case for ETOPS flights (i.e. long overwater/“over-uninhabited-area” flights) - if the normal trip fuel is not enough for diversion after cabin decompression, additional “decompression fuel” is carried.

@FallenUser The "10,000 feet" statement is sourced in the cited article as coming from "a reader's comment" and it's arguably wrong. Up to 24,000' without an emergency O2 source for the passengers is okay. (Not that it's a great plan with the masks all hanging like that.)

@FallenUser I'm sorry, could you highlight the sentence where it says that it is illegal in China?


text:
Will Aerodiesel engines replace Avgas engines?
I've been following aviation for the last 20 years and have seen particularly since the last 10 a lot of talk of aero-diesel engines for light aircraft. The idea being they're cheaper to run using Avtur rather than expensive Avgas.
I'm surprised that in 10 years since I first really saw this mentioned not much has changed. There are a few new aircraft available and retrofits, but by and large engines are still traditional in new aircraft.
Is it likely that aero-diesels will replace traditional aircraft engines any time soon?

I feel like this is all opinion based... After 10 years, who knows what could happen?

Things don't change very quickly in the world of light GA. Many of the light airplanes flying today were built between the 1960s and the 1980s (at least that's the case here in the U.S.) When you can buy a used plane for $20k - $30k or a new one of a similar model for $300k, it's not hard to see why a lot of people opt for older used planes. Also, the cost of developing and certifying new hardware is often prohibitively expensive compared to the relatively small demand.


text:
Who is responsible for maintenance and eventual design changes for planes that the manufacturer has gone broke?
Sorry for the wrong vocabulary, I am not someone from the aviation industry.
There was an incident (?) in a Foker 100 in Australia. Let's now imagine that there is some project change that has to be done. Foker is out of business for years now, who is responsible for doing this change? And paying for it?
A similar question also applies: where do aircraft parts that have to be substituted come from, so planes are still able to fly?

Those divisions of the company that manufactured parts and carried out maintenance and repair work were taken over by Stork N.V.; it is now known as Stork Aerospace Group. Stork Fokker exists to sustain remarketing of the company's existing aircraft: it refurbishes and resells F 50s and F 100s, and has converted a few F 50s to transport aircraft. Special projects included the development of an F50 maritime patrol variant and an F100 executive jet.

Ok, thanks a lot!


text:
How many holds to be IR current?
14 CFR 61.57(c)(1)(ii) requires "Holding procedures and tasks" to be IR current.

Within the 6 calendar months preceding the month of the flight, that person performed and logged at least the following tasks and iterations [...]
(i) Six instrument approaches.
(ii) Holding procedures and tasks.
(iii) Intercepting and tracking courses through the use of navigational electronic systems.

I have seen some people interpreting this as:

you need to fly one hold every 6 months
you need to fly at least 2 holds every 6 months (because of the plural "procedures")

I guess flying a hold involves several procedures, including entry into the hold, flying in/out bound, turning procedures etc. and each of them consist of several tasks. So 1 would seem to work, but everybody seems to have their own opinion on this.
Is there a definite official guidance on whether you need one or two holds in the past six months to be current?

@ymb1 Seen that thanks - it explains what qualifies as a hold but not how many are required.

Point 3 says "navigational electronic systems", that doesn't mean you have to use more than 1 navigational electronic system...

According to this AOPA article, quoting the FAA person who actually wrote 61.57, it’s one hold:

When paragraph 61.57(c)(1)(ii) [i.e., "Holding procedures"] was written it was not intended to mean multiple holdings. The emphasis is on the word procedures. There is more to holding than just flying circles in the sky. There is the planning for holding, slowing the aircraft down prior to entry, entering the holding pattern correctly, correcting for winds, establishing the one-minute inbound leg, etc. That is why I wrote it as 'Holding procedures.' So no, I never intended it to be multiple holdings.


text:
Why do police helicopters always fly in circles?
Why do police helicopters always fly in circles while news helicopter hover in one spot? Is it to avoid criminals opening fire at them, or is it because there is a difference between the two types of helicopters?

Same question: Why do police helicopters fly in circles instead of hovering?. However, I agree with others that police helicopters can just hover. I have seen them just floating above a point during street protests.

@Energizer777 It's amazing how many people who don't know what they are talking about comment with authority.  One of the answers in that question includes "auto-rotate were the rotors spin in the other directions".

This is just a comment because I have no references, but it seems to me like there's a strong advantage to being on the move because it is harder to dodge the helecopter's lights if they are constantly changing angles.

News copters want to deliver a stationary image whenever possible as it's what they want to show viewers. The police don't care about that.

I've never seen news helicopters hovering... @GdD I don't think stationary images has any relevance, as all news choppers I have seen have a stabilized camera which produces very steady images under most conditions.

In Boston where I live the police helicopters and the news helicopters all circle and rarely hover stationary. I have seen a police helicopter stationary over the harbor, but never over the city. Sometimes the helicopters over a newsworthy event look like a Lufbery circle, and I assume they circle because stationary craft could easily drift into collision.

Police helicopters don't always fly in circles. I've often seen my local police helicopter hovering. There are a few reasons you might have seen police helicopters flying in circles:-

Not all types of helicopter can hover out of ground effect. It might be that the police helicopters in your area can't hover at the heights they work at, with the weight of their usual equipment.
Helicopters are safer if they're moving forwards than hovering. In the event of an engine failure, they can autorotate to land safely, but they need to have some airspeed to be able to do this. The required airspeed depends on height (see the chart in the linked question), so if your news helicopters are usually operated higher up than police helicopters, they may be able to hover safely. (This is quite likely, as police aircraft are exempt from the 500 ft rule and the 1000 ft rule, while news helicopters are not.)
Police helicopters often circle to search an area for a fugitive or casualty using thermal imaging cameras. News helicopters tend to either follow a vehicle or hover to get a steady camera shot of a news event.

I also believe there is a risk of vortex effect that makes one lose all lift suddenly when hovering (in no wind situation).

@v.oddou Vortex ring state doesn't just happen randomly in a hover. It can be caused by descending too fast with too little horizontal velocity, or by extreme downdrafts. See http://aviation.stackexchange.com/questions/16108/how-do-modern-helicopters-tackle-vortex-ring-state

Could you give an example for a helicopter that is not able to hover?

@bogl Could you ask that as a new question? That way anyone can answer and you'll probably get some illustrated examples.

@DanHulme Helicopters are not subject to  the 500 ft rule, please see FAR 91.119. News helicopters can circle or hover just as low as an LE helicopter.

There is little difference between the helicopters used by news crews and police. It is their operation that differs.

Flying instead of hovering helps minimize the chance of getting shot. There has been incidents when law enforcement helicopters have been shot down.
Forward flight (at low speed) uses less power compared to hover. This reduces the engine load and increases the time spent over target.
Police helicopters fly pretty low compared to the news helicopters. Hovering for long periods can result in one settling in their own downwash. In this case, the altitude can help news helicopters to gain forward momentum to get out of this. As this option is not available to police helicopters, it is better for them to fly in circles.

Would another possible issue be a desire to avoid making viewers dizzy?

@supercat: I suspect that is the actual answer. The question is backwards: flying forward is safer and more fuel-efficient than hovering out of ground effect, so the question should rather be: why would a news helicopter not do that? BTW: I don't know how feasible it is to use a helicopter as a sniper platform (outside of Hollywood, that is), but in that case, they'd probably be hovering, too.

From what i know USCG use helicopter as firing platform (not to accuracy of sniper though).

Helicopters have been used in combination with snipers, but it's less precise than firing from a stable non-vibrating object like a building or elevated terrain

As far as I'm aware you can only have settling with power (vortex ring state) in a descent of more than 300-500 feet per minute with significant power applied. Not being in a hover

Hovering for long periods can result in one settling in their own downwash - no it can't.  In order to settle with power, you must be descending and if you're descending, you are not hovering.  You can hover until the fuel runs out, you get cramp or the next person needing the helicopter starts waving their arms in strange patterns.

Often the police helicopter is out looking for the target.  A circle is a search pattern.   News typically has a known target they just need a shot of.

Police helicopters search, follow, and chase things. You need to be moving to do this.  News media don't do those things.


text:
How to calculate the stability derivative of Pitch Moment with respect to AOA rate?
How to separate the stability derivatives: $C_{M\dot \alpha}$ and $C_{Mq}$.
I can imagine a $q=const, \dot \alpha=0$ movement like this

But what a $\dot\alpha=const, q=0$ movement would be look like?

What is the actual question here? Is what the $\dot\alpha = const, q=0$ movement looks like, or is it how do you derive $C_{M\dot\alpha}$ and $C_{Mq}$ from flight test data, i.e. what manoeuvres do you need to perform to acquire the necessary data?

@DeltaLima I mean what the movement of $\dot \alpha=const,q=0$ would look like

Wow, one  year and 16 days to respond to a clarification question... Welcome back!

$q = 0$ means no change of pitch angle. You also have a constant rate change in AoA ($\dot{\alpha} = const$).
The difference between the pitch angle ($\theta$) and AoA ($\alpha$) is the flight path angle ($\gamma$).
$\theta = \alpha + \gamma$
From this it follows that the flight path angle changes opposite to the change of change of AoA.
$\dot{\gamma}=-\dot{\alpha}$
This is not a manoeuvre that will happen in real life. An approximation of such a situation can happen when after a brief but strong pull-up the pitch angle is kept constant while the path angle of the aircraft is still increasing. The rate of change of the AoA will not be constant in such a manoeuvre.

Why not in real life? If the aircraft slows down so lift decreases, it will accelerate downwards without changing pitch. Voilá, the angle of attack increases and the pitch stays constant.

@PeterKämpf I agree with what you say and thought of that situation as well, but it does not change the angle of attack at a constant rate as per the original statement.

@Federico Thanks for the correction, I'll buy you a beer soon :-)

Depends on the rate of speed change. Yes, this is artificial, but so is the condition of a constant angle of attack change. By adjusting the speed, you can tailor the downward acceleration and thus the AoA rate.

If you add wind (a gust), the sought-after situation would be flying into a vertical gust. If the vertical wind speed changes linearly with the distance flown and the aircraft has sufficient inertia and/or low static stability, you get no pitch movement and a constant angle-of-attack rate, albeit only for a short time.

Thanks, I can imagine the situation. Adding wind is really helpful to make an intuitive image.


text:
Do laser Christmas lights have the same dangers as laser pointers aimed at planes?
With new laser Christmas decorations like this one, a laser appears to shine in patterns on a house for a decorative effect.  If this is aimed so that some of the beams miss the house and go over it into the sky, could they cause problems to planes?  Would the effect be similar to a laser pointer, and could the person with the decoration be prosecuted as shining a laser at a plane?

A fine beam is needed do draw patterns. A regular led would require a moving focusing mirror, something expensive.
There is a non-standard warning: "Laser aperture, Max output < 1 mW" near the leds:

As the leds are in the visible spectrum, this is equivalent in power to a Class 2 laser.

A Class 2 laser is considered to be safe because the blink reflex
  (glare aversion response to bright lights) will limit the exposure to
  no more than 0.25 seconds. (Wikipedia).

This image shows the effect of a 5 mW laser light. You may use it for a 1 mW laser light by dividing the distances by 5:

Adapted from original on Wikipedia
or from the original site:

A 0.99 mW Class 2 laser beam can temporarily flashblind a pilot or driver, causing afterimages, within 240 ft (73 m) of the laser.
It can cause glare, blocking a pilot or driver's vision, within 1050 ft (320 m) of the laser.
It can cause distraction, being brighter than surrounding lights, within 2 miles (3.2 km) of the laser.

The above distances are for a tightly focused beam, which is probably not the case of an inexpensive laser device. The effects of a more diverged beam are weaker.
More info: This hazard chart.
To sum up:

Most countries have regulations for dazzling a pilot, regardless of the mean and power used, it's the effect that is taken into account.
A class 2 laser led couldn't blind someone unless the person is at a few meters (15 m for temporary blindness and 3 m for eye damages, if the eye is forced to stay open).
As the class is not mentioned on the label, I would be careful when seeing this item.
There are actually 2 laser leds, a red and a green, is it 2x1 mW or 2x0.5 mW? In practical the lasers probably can't illuminate the same eye, due to the divergence of their beams.
To draw a pattern a laser light must move, so the time of illumination of a point is reduced when the device is working correctly.

Yes, they do
The FAA will not tell you how to decorate your yard, it is not going around and regulating yard displays, however if your display is distracting pilots you will get an unwelcome knock on the door. If you are pointing them up on purpose though, it's an $11,000 fine and possible jail time.

These types of lights are unlikely to affect an aircraft.
The lasers in Christmas projectors are 1 milliwatt lasers. This power of laser is too weak to make a noticeable distraction beyond about 500 feet, so it would be unlikely to affect a plane. You can test this yourself by buying a 1mw laser pointer from Staples or other stationary store and having a friend shine it into your own car parked at the other end of a football field or wherever to see the affect. Beyond a certain distance the light attenuates into invisibility.
Generally speaking the minimum power laser needed to affect an aircraft is a 5mw laser which can reach several thousand feet and affect aircraft which are landing.
Another thing to realize that the odds of a house light accidentally pointing at an object far away is pretty remote. The typical divergence angle for a laser pointer is around 0.5 degrees, so assuming a hemisphere the random chance of an intersection would be 1 in 82,506.
Usually for a laser to present a distraction to an aircraft it has to be deliberately pointed and the person needs to be near the approach path of the runway.

A guy I know who flies helicopters was momentarily blinded by a stray beam.
He confirmed the cause as laser christmas lights when he drove by the residence he was flying over.


text:
Anybody know the reason for the ‘Star Wars’ themed STARs into KATL?
There are new RNAV STARs for arrival into Atlanta Class Bravo for KATL.  Does anybody know the reason a Star Wars theme was chosen for waypoint names eg TYFTR, WOKIE, NWHPE, etc?

Probably as simple as, somebody likes Star Wars.  There are a lot of similar points on one or two of the arrivals into KAUS as well.

What's life without whimsy?

It's a long established practice for the procedure developers to build the procedures around a theme. It helps with coming up with the necessary distinct pronounceable names for the waypoints.
My personal favorite is the RNAV (GPS) RWY 16 approach into KPSM. Flying from the IAF to the MAP and the Missed Approach Holding waypoint you fly:
ITAWT --> ITAWA --> PUDYE --> TTATT --> IDEED
which mimics Tweety Bird's comment when he sees Sylvester the Cat chasing him (for those readers who didn't grow up watching Warner Brothers cartoons).

Interesting that that arrival was in Portsmouth NH.  I had thought that the naming was a regional thing such as the FRDMM4 arrival into Washington, DC in honor of Sept 11, 2001 or the old IVNKA1 arrival into KPBI because of a certain notorious resident who owns property there.  I was curious if there was some sort of connection between the Atlanta area or airport authority and Disney/Lucasfilm that inspired the STARs.

Instead of choosing random names, some sort of a theme helps to come up with names one could easily pronounce, and because why not? :)
Another example:
PIGLT FIVE arrival into KMCO (Orlando Intl.)
i.e. HKUNA — MTATA — JAZMN — JAFAR — RFIKI (Disney references)
This article from aerosavvy lists a few more interesting ones.

Well that one makes sense since it is an arrival into Orlando, home to Disney World

I was just trying to figure out if there was a connection between Atlanta and Disney or Lucasfilm

That is definitely one possible explanation. Those arrivals are fairly recent additions – probably within the last year or so


text:
Are there any negative health indications from getting sea legs?
I recently completed a long XC and upon completion I suffered a bit from sea legs. The XC was for the 250nm requirement for my instrument rating and was performed in a C172. It took about 3.5hrs
 to complete and there was a short ~30min pit stop a little before hr 3 of the flight. I was under the hood the entire time except for takeoff and landing. The air wasn't particularly smooth as
we were only @3-4k MSL and surface winds were in the teens.
I completed the flight in the early evening (~8pm) and feel like I suffered from sea legs, a small wobbly sensation of constantly moving, until I went to sleep (~11pm). I'm guessing the sensation would have lasted longer had I not gone to sleep. The sensation only occurred once on the ground and after completion of the XC. I don't recall having the sensation during the pit stop.
I'm curious how or if I should be concerned about this.  I do not suffer from in flight nausea, motion sickness or anything like that.
I have never had this happen on shorter, 1-2hr, training flights.  I've never had this happen when flying on a commercial jetliner.
I remember experiencing the same sea legs sensation on another long ~400+nm XC years ago. I flew a plane with an autopilot that rocked back and forth as it held the heading and I attributed the post flight sea legs to the oscillations of the autopilot.
Is this common for pilots to suffer from sea legs post flight? If it is common, how long does it last?
Is this an indication of a pending medical issue or an issue that might be of concern?
I'm curious if this is an indication of something of concern in my inner ear.
Are there any concerns with bringing this up with my AME? I have to get my medical renewed this year.

NEVER bring it up with an AME unless you feel that your condition could affect the safety of flight and that you should never fly again.

this is a medical condition called mal de debarquement syndrome or MdDS. Do a search on line, you will find literature on it. I developed a severe case after flying from Denmark to Los Angeles in a DC-7C in 1960 which would flare up anytime I was in an enclosed space, like a clothes closet or stairwell. it did not subside until 10 years after that flight.

This question is intrinsically somewhat a matter of opinion isn't it, as many medical issues are.  Still it's too interesting to throw away.  A more cut-and-dried question for Stack Exchange might be along the lines of "would an AME disqualify me if I shared the following observation with him"--

Key point --

The sensation only occurred once on the ground and after completion of
  the XC.
until I went to sleep (~11pm)

If you have no further symptoms or at least no more severe symptoms, then I see no reason to be too concerned about this, and certainly no reason to bring it up with an AME.  (But see added comment below re long-term career plans.)  Interestingly, after hours spent feeling certain repetitive sensations, whether due to to thermalling flight in a hang glider or sailplane, or bobbing up in down in a kayak, I have sometimes noticed a sense of the motion continuing later, but ONLY while laying down prior to falling asleep in the evening. These sensations may have been more related to the repetitive forces felt by the skeletal-musculature system (increased G-load, rhythmic variation in G-load), than to the vestibular-occular system, so perhaps they were fundamentally different from what you are describing.
Comment incorporated into answer--

this is a medical condition called mal de debarquement syndrome or
  MdDS. Do a search on line, you will find literature on it. I developed
  a severe case after flying from Denmark to Los Angeles in a DC-7C in
  1960 which would flare up anytime I was in an enclosed space, like a
  clothes closet or stairwell. it did not subside until 10 years after
  that flight. – niels nielsen

A google search on that term produced other similar results of symptoms lasting many years-- remarkable.  Some rather interesting ideas may be found about just what is going on here, and how to treat it.  See for example https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5418223/

We reasoned that if the notion of MdDS caused by maladaptation of the
  VOR holds true, a reversed VOR stimulus should mitigate or even cure
  the MdDS symptoms. Based on these findings, a treatment for MdDS was
  devised that consisted of rolling the head at the frequency of body
  rocking, while the subjects viewed a slowly moving visual surround.
  This postulate was tested in 24 MdDS patients employing cross-axis
  visual coupling (1). The results were promising: 75% of the MdDS
  patients had significant improvement in a 12-month observation, which
  was the first successful treatment of this debilitating illness.

(Front Neurol. 2017; 8: 175.
Published online 2017 May 5. doi: 10.3389/fneur.2017.00175
PMCID: PMC5418223
PMID: 28529496
Treatment of the Mal de Debarquement Syndrome: A 1-Year Follow-up
Mingjia Dai,1 Bernard Cohen,1 Catherine Cho,2,3 Susan Shin,1 and Sergei B. Yakushin1,*)
It would appear that there are enormous gradations in how severe this sort of thing can be.  If you are simply feeling some sensations on the ground, after a flight, and they are gone the next morning, it would seem both un-necessary and unwise to mention them to an AME.
I would say that occasionally-recurring symptoms at the level you've reported might be reason to think twice about a career as a professional pilot-- lest the symptoms get worse-- but still not something that would suddenly and significantly compromise your ability to fly safely on any given instance, and therefore not something that you should feel compelled to report to an AME.
Obviously I am speaking from the viewpoint of what is in your best interest as long as  it does not significantly conflict with safety.  An AME might well give a different opinion.


text:
Who was the US military pilot who crashed his plane and was accused of selling it to the Russians?
There was recently news regarding a story of a former US military pilot who crash landed his plane somewhere in the southwest. He was accused of flying the airplane to Mexico to deliver it to the Russians. The Russians then supposedly dropped him off in the desert from which he walked back to civilization. I believe this was Post WWII & Korea. The pilot lived with the accusation his entire life despite claiming his innocence despite the plane never being found.
The wreckage was just recently found, vindicating the pilot although he had passed away some years ago.
Any help would be appreciated. My google searches came up with many defection stories and red herrings.  I know the story is recent. I just can't find it.

@ChrisStratton, I believe that you are correct. I knew I recently heard about it and it appears I read about it here: https://www.planeandpilotmag.com/article/the-case-of-the-missing-t-33/  For some reason I thought that the reason I had read about it recently was due to the recent publication of the article.

According to the relevant Wikipedia page Lt. David Steeves of the USAF bailed out of a T-33 trainer in 1957 near King's Canyon National Park in California and was not found until 52 days later.  When the crash site could not be located Steeves became the subject of various rumors, despite the T-33 being a relatively unimpressive trainer aircraft.
Steeves left the air force after the incident and reportedly conducted some of his own searches on the ground and in the air before being killed in an unrelated crash in Idaho in 1965.
A canopy located in the park in 1977 was as identified as being from this aircraft in 1978 but the remainder has not been found.  While no developments appear to have taken place in the story since then, it has occasionally been the subject of articles over the years.


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Portable radio for cockpit communication
I am a private pilot based at Van Nuys airport in Los Angeles. Recently due to traffic congestion we have to hold short for extended periods of time, 20-25 minutes.
I thought to get a portable radio to be able to turn the airplane engine and master switches off and save on the rental fee and the constant breaking.
Any recommendations about the specifications of a say 5-10 nautical mile range portable radio is appreciated.

You won't significantly run a battery down with everything off but a comm radio turned on and listening in. GA comm radios are generally around 5 watts transmitting, much less when in receive mode, a couple watts at most.  You're talking a fraction of an amp current draw.  A 12 vdc 25 Amp Hr battery will take a day or two to run down. And you're starting a warm engine.  If a battery can't start a warm engine after running a radio in receive mode for 30 min with 5 seconds transmitting, that's a battery that's done, and whether you had the radio on or not will have made little difference.

Not to say you shouldn't buy a backup radio if you fly from a place like Van Nuys; you should have one anyway if you fly in any kind of busy airspace.

@John K. they charge the hubs time. not the tachometer.

Hobbs is activated by oil pressure.  It only records while the engine is running.  Next time you shut down, leave the master on and observe the meter for 6 minutes and you'll see it stopped counting.

@JohnK Note that some greedy FBOs rewire the Hobbs to the master for such reasons.

Yikes!  If I caught someone renting a plane to me doing that, they'd get an earful from me and I certainly wouldn't rent from them knowing I'm paying 2 dollars a minute for the use of the battery.

Buy a portable, handheld transceiver. It can very well come in handy in an emergency. Especially one with GPS, VOR, and ILS. Make sure you get a headset adapter with it.
Do not use it to save money during a long hold-short request. Turning the engine off after you have done your run up is not recommended. It is defeating the purpose of the run up. A better technique is to get your flight following discrete squawk code and/or clearance delivery electronically, by phone, or by handheld before turning on the Master and engine.


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Did the contemporaries of the F-14 also have automatic swing wings?
The F-14's Central Air Data Computer was a innovative design, using microprocessors for its implementation. This computer was used to control the swing wings of the F-14.
How did the contemporary aircraft to the F-14 achieve the same functionality? There were many variable-sweep wing aircraft constructed in the 60's and 70's such as the Mig-23 and the Su-17.
References:

https://en.wikipedia.org/wiki/Central_Air_Data_Computer
http://firstmicroprocessor.com/documents/ap1-26-97.pdf

If you think that was innovative, the RIO's target display system was based on two Intel 8088 microprocessors and no math coprocessor(s). All the math was table-driven. Think about it -- tracking up to... what was it... 12? targets at the same time with a slide ruler. That was not coding, that was art. :) (Yes, I worked on this subsystem at Hughes Aircraft Company's Radar Systems Group at El Segundo, but I worked on self-test code verification.)

@JuanJimenez Even now, many programs for embedded systems will use lookup tables for trigonometric functions and the like, so it's not a lost art just yet.

@JAB Warms my dark PL/M heart to hear that. LOL!

I don't think there were any other variable-sweep aircraft with automatic sweep control. Later versions of Tornado had the feature but it was reportedly hardly ever used. MiG-23 and Su-17 are not directly comparable to F-14: these were (conceptually) cheap mass warplanes, unlike the very sophisticated F-14. Su-24 (a counterpart to F-111) had a high level of sophistication, but its sweep control was still manual. Traditionally, control automation is not a Russian forte (in aviation - as contrasts sharply with Russian spacecraft!).

@Zeus - Soviet nuclear attack subs were also heavily automated at times. The famous titanium-hulled Alfa's had just 31 crew members (all officers). The entire rear half (reactor and machinery) was basically unmanned. This reclaimed a lot of space/weight (just 2300 tons surfaced). This light hull, coupled with a reactor more powerful those on subs twice its size, allowed the Alfas to push incredible speeds

@JuanJimenez: Are you sure it was the 8088?  The F-14 first flew in 1970 and entered service in 1974; the 8088 wasn't available until 1979.

Yes. You're thinking about the AWG-9 that used 8080 cpu's. The AN/APG-71 was the upgrade developed by Hughes in the 80's to replace the AWG-9.


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What are the differences between pre-transistor and transistor era jet engines?
What are the differences between pre-transistor and transistor era jet engines? Towards the end of the second world war, most of the air powers had started development on jet engines. From what I understand, all of this predated any practical application of transistors. The transistor era obviously gave way to integrated circuits and microprocessors. On any piston engine I can usually spot the evolution of components because I'm well versed in the operation of piston engines. For jet engines it is less obvious to me, there are for example differences between turbofan and turbojet engines. I superficially understand the difference between them but I've never even operated one, much less been hands on with it.
What are the significant changes from pre-transistor to transistor-era jet engines? I've never seen vacuum tubes used on jet engines so it doesn't seem like it is just a matter of "they replaced the tubes".

The primary advance in the transistor era is the FADEC. It's essentially a fly-by-wire throttle, incorporating rate limiters to key engine inputs such as fuel flow rate, and performing automated monitoring tasks to reduce flight crew workload. Prior to the FADEC system, sufficiently complex jet aircraft required a crew member, the flight engineer, whose primary task was monitoring and maintaining engine performance in flight. Additionally, jets without a FADEC require particular attention paid by the pilot(s) to the rate at which the throttle is adjusted; you can't just firewall it for a takeoff as that will cause a compressor stall.

So, FADEC would have prevented accidents like this one involving an F-14? https://www.youtube.com/watch?v=prEFjYrAJuw

Probably not. The F-14 was notorious for compressor stalls, especially with its original TF30 engines (however only the F-14As had these; Bs and Ds in service by 1995 used more reliable GE F110-400 engines). You'll notice transonic shockwaves developing several times as the plane flies by, which indicates to me the aircraft is holding near Mach 1, and that's a very dangerous place for fighters with large rectangular intakes to be as leading-edge shockwaves can enter the engine and cause wide variations in compressor pressure, thus causing wide swings in fuel-air mixture.

The vibration levels on jets preclude the use of vacuum tubes. They may work if everything is fine and all rotating parts are well balanced, but once you have a defect and an out-of-balance condition, the electronics would fail quickly, just when you need their help the most. Especially the afterburner adds a lot of vibration when it works as designed already.
Earlier jet engines used hydraulics for adjusting things like compressor vane angles (litte stationary "wings" positioned between the rotating discs of the compressor, which help to convert rotational flow energy into compression) and the nozzle size. Like on piston engines, the amount of adjustment was much lower back then, and all adjustments were done mechanically.

Vacuum tubes can be build to withstand vibrations. The proximity fuse works with vacuum tubes inside a projectile shot from a cannon.


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Has an aircraft carrier ever assisted a civilian passenger flight?
Has an aircraft carrier ever assisted a civilian passenger flight? I am trying to find an example of this. It could be as extreme as a small plane landing on aircraft carrier in an emergency. A simpler example would be a civilian passenger plane requesting navigational assistance from an aircraft carrier over VHF communications.

Well, we had a "small plane land on an aircraft carrier" question just recently: https://aviation.stackexchange.com/questions/33170/a-daredevil-attempt-to-land-on-an-aircraft-carrier

In your scenario the pilot would ask either ATC on the assigned frequencies, or call on emergency frequencies not attributed to a specific aircraft or ship when ATC is not reachable. As the pilot cannot know what are the radio frequencies used by carriers, or where are carriers located, they cannot ask carriers assistance, or know that some ship is within radio range. Maybe also look at ETOPS.

@GregHewgill I think it's a duplicate of that question but this is worded a lot better.

@mins I believe you are correct but it does not preclude my question. If a passenger flight had failures of satellite and HF communications over the Pacific, the pilot would use VHF on emergency frequencies to request assistance from anyone.

@EricUrban, from the wording of your question I am unclear if you are also interested in cases where a carrier (particularly the helos in the air wing) have assisted with a civilian aircraft that has crashed (via SAR).  Also, are you only interested in carriers, or more generally warships?

Possible duplicate of Has an unauthorized civilian ever tried to land on an aircraft carrier?

It has happened before.
There are definite cases of light civilian aircraft landing on a carrier, the most well known was in April 1975 when South Vietnamese Air Force Major Buang Lee landed an O-1 Bird Dog on the deck of the USS Midway trying to escape with his wife and 5 children from the fall of Saigon to North Vietnamese forces.
Landing aboard an aircraft carrier presents several difficulties, namely limited landing roll and available deck space.  A Nimitz Class carrier is just shy of 1100 feet in total length and 250 ft at its widest point.  The designated landing area or LA is approx 75 x 650 ft.  The ship is capable of steaming at 30+ kts so it can develop quite a bit of wind over the flight deck reducing the effective unarrested landing roll.
Then you need to stop the aircraft once it's on the deck.  Carrier based fixed wing jets use tailhooks designed to work with the arresting gear and reduce the landing roll to about 330 feet.
The largest carrier aircraft in service were the Douglas A-3 and the Grumman C-2 and E-2, each one about the size of a regional jet.  The largest aircraft which I am aware of which made a successful unarrested landing was a Lockheed C-130.
Large commercial aircraft like a Boeing 747 or an Airbus A-380 simply cannot fit on the deck without the wings clipping the island or other deck antennas, etc, not to mention requiring landing rolls of over 3000 ft even in the most extreme short field attempts.
So HAS a carrier aided a civilian passenger flight via recovery operations?  Yes but only with small aircraft.  COULD a carrier aid in recovering larger commercial aircraft?  Possible depending on the size of the aircraft, its loadout and fuel state and performance capabilities.

It should be noted that the O-1 was not much of a civilian plane (technically). The O-1 is a Cessna L-19/305 for military use not much bigger than a C-170 except it had a big engine (213hp), it was stolen by the Major to defect. It cost $10m in lost equipment for that carrier landing by Maj. Buang. Most (US) carriers also have a "crash net" that can stop pretty much anything that can fit on the deck.

You're not stopping a 747 with a carrier barricade.  That's not gonna happen.

No, but it won't fit on the deck either...


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Why are airplane passengers boarded from front to back?
Last time I boarded a flight (Boeing 717 with Delta, with assigned seats), the passengers were split into 4 groups and allowed to board the plane in an order that filled seats from front to back. I can understand why first/business class could go first, since they pay more. But the rest of the passengers pay the same rate.
Wouldn't it be more efficient to let the passengers that sit in the back board first? Or maybe even to just let everyone board in random order.

Boarding schemes differ per airline. It's also interesting that you say you boarded front to rear, as Delta typically boards back-to-front.

A number of airlines (including Delta) use a 'zone boarding' procedure. In short, the first/business class board first, followed by 'blocks' of passengers, usually from rear to front (In your aircraft, this was apparently done the opposite way).
However, it is important to point out that the back-to-front is not very efficient, atleast if you take into account the time taken to seat everyone. Experimental evidence indicates that the back to front method is one of the least efficient of all the boarding methods. The table below shows the time taken for 72 people to board a (mock) airplane using various methods.

Table from Experimental test of airplane boarding methods by Jason H. Steffen and Jon Hotchkiss
The basic problem in following back-to-front method, according to the author is that it is a serial process:

The passengers would rush into the cabin, proceed toward the back—and come to a screeching halt as the first one or two passengers stowed their luggage. The first 30 passengers (the back five rows) would take up nearly the entire length of the cabin. The rest of the line has simply moved from the airport gate into the jetway or cabin—and it moves no faster.
... In this case, only one passenger at a time is seated. The aisle in the airplane isn’t used effectively.

There has also been studies on optimal boarding procedures, which indicate that optimal procedure is different from what any airline follows.

There was a mythbusters episode that simulated boarding an airplane using various methods in vogue. Making allowances for the lack of scientific rigor, the results of boarding 173 passengers were pretty much the same- back-to-front is the slowest method.

Image from mythbustersresults.com

I remember not trusting the Mythbuster's methodology. They showed one boarding with each of those methods, all using the same people (I do think they changed people's seat assignments from one test to another). It seems likely that boarding times would improve with practice, so the first attempt (back-to-front) would have done better if it had been done last.


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Are the N1 and N2 tachometer generators installed on 727 / JT8D engines line replaceable units?
Each JT8D engine on a 727 has two tachometer generators as part of the engine tachometer system to measure N1 and N2 speeds. Are these generators line replaceable units? How long would replacement of just one generator take?

Usually, they are. In case of the JT8D engine, the N1 tach generator (inside the nose bullet), atleast is a LRU. The AMT Society even has a competition which involves the removal of N1 tacho generator.


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Why does the CF-100 autopilot have a toggle switch labeled 'ATTACK'?
I took this photograph of the autopilot controls inside the cockpit of a CF-100. There are a bunch of switches for the autopilot. Why does one have a label of "ATTACK" and what did turning it on/off do?

I have absolutely no reference and 100% guess on this: is it possible that "attack" means "angle of attack", not "drop bombs or file missiles" ?

It automatically guides the aircraft to an attack (target intercept) based on the flight computer inputs and radar. I can't find a really good source on it, but it has been around since the 1960's. See this Google Books Excerpt

@Kevin File them under M for Missile?

Automatically guides the aircraft to a preset set of coordinates or a radar return?


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What's the fastest atmospheric missle that demonstrated a useable radome?
I read that

DoD has stated that its hypersonic boost-glide missiles will be able to emit and receive radio signals, such as those from the Global Positioning System (GPS). That statement implies that portions of a missile’s glide body where transmitters and receivers are located will remain below the temperature threshold for plasma formation in the surrounding air, as DoD’s modeling suggested might be the case. [...]
Although the sensitive electronics necessary for communications must be adequately shielded from a missile’s exterior temperatures, the missile needs to have windows (called radomes) through which signals can be emitted or received. The materials for such radomes are inherently difficult to develop because they must allow radio frequency or infrared radiation to pass through them while also providing significant thermal shielding.

What's the fastest atmospheric  flight that still demonstrated a useable radome,  e.g. for GPS or remote command etc.?

"they must allow [...] infrared radiation to pass through them while also providing significant thermal shielding", is that possible?

That would be the Space Shuttle.
A more detailed, in-depth overview of the various antennae and their radome/window design can be foud in this document: INVESTIGATION OF
HIGH TEMPERATURE ANTENNAS FOR SPACE SHUTTLE
The Space Shuttle's highest heat flux was reached at 63-68 km altitude, which corresponds to ~5,500 m/s velocity during the reentry phase. However, this did result in a temporary communications blackout.
This leaves about 3,300 m/s as the highest airspeed at which a production hypersonic vehicle, namely the Space Shuttle, could successfully communicate while in the atmosphere.

Heat flux can be mapped to altitude via the graph below.

The paper this graph comes from, Plasma-Radiofrequency Interactions Around Atmospheric Re-Entry Vehicles, also mentions experiments that have achieved communications at higher velocity. "As a general rule, the steeper the trajectory ... the lower is the altitude at which recovery occurs."
The RAM-C reentry vehicle used a very simple blunt cone shape. As the image shows, most of the vehicle was covered by a plasma sheath, so the nose was left as an antenna window. It still had a blackout phase above 5,200 m/s. This gives about 5 km/s as the experimental limit for very restricted communications.

For missiles specifically, currently the one operational hypersonic, the Zircon at 2,650 m/s, is slower than the Space Shuttle. DoD designs target further lower speeds, in favor of range, so it's very likely that they will be able to communicate.


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What is a straight wing on an airplane?
What is a straight wing? I need this for my science project. I have looked for other answers on this website, but I cannot find any. Please help!

It's a wing that is not swept.

What is a straight wing on an airplane?
A straight wing is a wing that is not swept (see the images).

Straight and swept wings

Unswept, straight wings

Swept wings

Please edit your answer to include the attributions for the images.


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Is it quiet in the cockpit when flying faster than the speed of sound?
Is it quiet in the cockpit when flying faster than the speed of sound or is the noise from air resistance just too loud?
Similar question: is it quieter at the front of an airliner flying faster than the speed of sound because the engines are behind you (comparing to the rear of the plane)?

There may also be an answer to your question here, but as an aside, if you're at all interested in anything about the Concorde: http://omegataupodcast.net/2015/02/166-flying-the-concorde/

It is indeed quieter in the forward part of the cabin, but already at subsonic speeds, because the boundary layer is thinner and produces less noise. This is audible in subsonic jets; supersonic jets have too loud engines to let you pick up such subtleties.

You should include the supersonic-tag. I wanted to edit your question, but obviously a bug occured that an edit must be at least 6 charachters, although the word "supersonic" has more.

No, for two reasons:

Speed of sound is always relative to the material the sound propagates in; it moves faster through denser media, such as the aluminum structure of the aircraft itself. The aircraft's structure moves with the aircraft, so the vibrations of the jet engines will be transmitted to the cockpit/cabin at the same speed as when the airplane were flying subsonic.
The air inside the cockpit is still, and so sound within this local bubble is unaffected by the speed of the bubble itself (and so sound can travel faster than sound). If this weren't true then radios and other cockpit sound sources like alarms would be inaudible at supersonic speeds, which would be a major safety hazard and tactical disadvantage to supersonic flight.

You actually don't even have any significant aural cues inside the cockpit that you've exceeded Mach 1; you have to rely on your airspeed gauge to tell you.

The jet sounds the same inside the cockpit at M .98 as it does at M 1.3 .  The sound is still the same relative to your ears - everything is still in the same proximity to you.
I have flown at 1.3 and what does change is the airflow around the aircraft, changing the flight characteristics of the jet.  The handling of the jet becomes much more stable.  If your jet could aileron roll at 360 degrees per second subsonic, it now lumbers through a roll at perhaps 120 degrees per second.  Your only real indicators of going supersonic are seeing your pressure instruments (airspeed/Mach indicator and VVI) momentarily jumping as you pass M 1 and the fact that your shiny little rocket now flies like a 172, albeit considerably faster, higher and cooler.


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When has the SR-71 been used for civilian purposes?
I recently saw documentary about an incident in the 1970s that off-handedly mentioned that the Air Force had "loaned" an SR-71 to help with some search efforts. I assume by "loaned" that the Air Force took pictures where they were asked rather than hand the keys over.
Was this a regular thing or was it unusual?
How many times has the SR-71 been used for non-military purposes?

Are you sure they said "search efforts" and not "research efforts"? NASA used the SR-71s for some scientific research projects.

@DarthPseudonym - I added an answer with the one I heard about. I was hoping someone else would know more about it and other weird ones.

Would filming a movie about the military count as "non-military purposes"?  I'm sure there are examples of that.  Hollywood often "borrows" military aircraft and other vehicles.  (Generally they are still flown by actual military personnel, but the purpose is filmmaking, not military.)

@DarrelHoffman - yes, that would seem to fit. I assume this would be something out of the ordinary and not just filming some already planned mission. DId they use it in X-Men? lol

Yes, the SR71 was briefly reactivated during the 90s before being retired again in 1998. About the search efforts I don't know.
During the 1990s two SR-71 Blackbird aircraft were used by NASA as testbeds for high-speed and high-altitude aeronautical research at Dryden - Edwards AFB, California.
SR-71 activities at Dryden were part of NASA’s overall high-speed aeronautical research program and involved other NASA research Centers, other government agencies, universities, and commercial firms.
By the time the Air Force loaned the two SR71s to Dryden the center already had a decade of past experience with the Blackbirds. Three of the aircraft were flown at the facility between December 1969 and November 1979 in a joint NASA/Air Force program
The exact quote from the NASA website says

The aircraft included an SR-71A and SR-71B (the trainer version), loaned to NASA by the U.S. Air Force.

Perhaps this is the loan mentioned?
There's more information about the SR71s at NASA over here.
In fact even its very last flight was with NASA,

9 October 1999: The last flight of the SR-71 (AF Ser. No. 61-7980/NASA 844)

Thanks. I was wondering about other times like this. This obviously isn't the same as it's not in the 70s. I know what the 1971 use was for - I didn't want to bias or eliminate any other answers by mentioning it.

Yes I saw that, I was mostly interested in answering the second question,
"How many times has the SR-71 been used for non-military purposes?"

December 1971 - Washington - Search for DB Cooper
The FBI got use of the SR-71 Blackbird to search for signs of D.B. Cooper. Cooper hijacked a Boeing 727 from Northwest Orient Airlines on November 24th, 1971 during a flight from Portland to Seattle. In Seattle he was given $200,000 and 4 parachutes. During the flight to Reno around 9:00, he jumped off the 727's aft staircase over Washington.
A ground and air search didn't turn up anything so the FBI got use of the SR-71 to fly reconnaissance to find any signs of the parachutes or anything else.

On December 6, 1971, FBI Director J. Edgar Hoover approved the use of
an Air Force SR-71 Blackbird to retrace and photograph Flight 305's
flightpath and attempt to locate the items Cooper carried during his
jump. The SR-71 made five flights to retrace Flight 305's route, but
due to poor visibility, the photography attempts were unsuccessful.

Wikipedia/DB Cooper
On the DB Cooper documentary I saw, they mentioned that the SR-71 was used to search the area, implying it was the next day looking for him but it was about 2 weeks later, and they were looking for any signs.

NASA Space Shuttle pilots used SR-71 as a decoy when they practiced intercepting a landing space shuttle. This was before STS-1 when they were proofing their procedures prior to the first Space Shuttle flight. But of course, the SR-71 was operated by Air Force crews. Even if it was a one-or-few times thing i would classify it as a non-military operation.

What do you mean by "used as a decoy"? And who was intercepting what?

It appears to have been used to mock the shuttle lander because it could approximate some aerodynamic characteristics of the shuttle; see a post on the subject.

@Jpe61 NASA used chase planes to provide an external view to help the very fast, very ungainly Space Shuttle glide to a landing like a brick fired from a cannon. Example from STS-3. 6 minutes from landing the Space Shuttle would still be doing 2000 mph. See Landing the Space Shuttle Orbiter. Presumably they needed the SR-71 so the chase planes could practice intercepting something that fast.

Do you know where? Mojave? Somewhere a little more private like Groom Lake?